Coupling for frequency modulation discriminators



y 5, 1953 J. c. SPINDLER 2,637,809

COUPLING FOR FREQUENCY MODULATION DI ESCRIMINATORS Filed Nov. 1, 1947 MM. .ww Q 55 T "Ill INVENTOR.

JOSEPH C- SPINDLER ATTORNEY Patented May 5, 1953 COUPLING FOR FREQUENCY MODULATION DISCRIMINATORS Joseph C. Spindler, Chicago, Ill., assignor to The Rauland Corporation, Chicago, 111., a corporation of Illinois Application November 1, 1947, Serial No. 783,567

3 Claims.

This invention relates to improvements in the circuits of receivers of frequency modulated signals. More particularly, it relates to improved circuits, including improved double tuned transformer couplings, for discriminators of frequency modulated signals.

It is well known that the most Widely used discriminators, i. e. devices for converting frequency variations into amplitude variations and for thereafter detecting the modulating component, are of the type comprising a double tuned transformer coupling having a primary which receives a single input, and a secondary which separately feeds two outputs to two conventional detector circuits, and an in-series arrangement of the detector load resistors for algebraically adding together the voltages separately developed across them. For maximum linearity and symmetry of the response curve of such a discriminator it is desirable that the secondary of its coupling have a center tap located at its true electrical center and that the two halves of the secondary be equally coupled to the primary. It is the general practice to seek to obtain these conditions by close control of manufacturing accuracy, and to make little if any provision for adjusting a transformer coupling which has been inaccurately manufactured or has gotten out of adjustment. In usual practice the primary consists of a winding with a tunable condenser in shunt thereto. No means is provided for varying the inductance of this winding probably because of the fact that adjustments, such as movements of a core of high permeability, would in a conventional arrangement tend to unbalance the coupling of the primary to the two halves of the secondary. The shunt condenser is normally adjusted so that the primary is antiresonant at the carrier frequency and it is hoped that it can be left that way. The secondary ordinarily consists of a single winding which has a center tap and is in shunt to a variable condenser. It is customary not to provide any means for varying the selfinductance either of the secondary as a whole or of either of its halves. Instead manufacturing accuracy is again relied on, this time for locating the center tap at the exact electrical center. The shunt condenser is used for tuning the secondary to anti-resonance at the carrier frequency. Once a coupling of this type has been manufactured with any inaccuracy as to the location of the secondary center tap and/or as to the equality of coupling between the primary and the two secondary halves, very little can be done about it. The best that has been done is unsatisfactory. This consists of trying at random dif- I ferent combinations of detunings of the primary and the secondary in an effort to produce distortion of the discriminator characteristic which will exactly oppose and compensate for distortion therein produced by one or more of the inaccuracies mentioned above. The difiiculty with this method is thatwhile asymmetry of one of the two possible types may be used to compensate for that of the other, e. g. unequal coupling may be used to compensate for electrical displacement of the center tap or vice versa, neither asymmetry nOr any combination of them can be directly opposed in a clean-cut and predictable manner by detunings of the primary or secondary alone or in combination. Moreover, in neither case can the actual inaccuracy be directly corrected. For example an electrically off-center center tap cannot be shifted to on-center. At best its effects may be compensated for, and this is not certain. The lack of flexibility of their transformer couplings has been a serious shortcoming in discriminators of the prior art. Moreover, the fact that these couplings must of necessity include lumped capacitance in both the primary and the secondary (since tuning is accomplished by capacity variations) is a disadvantage at high frequencies where one of the primary problems is to keep down circuit shunt capacitance. The portion of this discriminator which is connected to the output side of its transformer coupling is well known. A detector circuit, such a a diode and a load resistor which is in series therewith and is by-passed by a condenser, is connected across each half of the secondary, the load resistors of the two detectors being in series. Energy is fed to the secondary via the inductive coupling between the primary and the secondary and, in addition, there is a direct connection from one end of the primary, usually over a direct current blocking condenser, to the center tap of the secondary, the reasons for this being well known. M

It is an object of the present invention to devise an improved transformer coupling in which the electrica1 position of a center tap of the secondary is variable after manufacture by simple adjustments.

It is a further object of the present invention to devise an improved transformer coupling in which the electrical position of the center tap" is readily adjustable after manufacture either to the electrical center of the secondary when the values of coupling between the primary and the two halves of the secondary are equal, or' to some off-center position to compensate for electrical effects produced when the values are unequal.

It is a further object of the present invention to devise an improved transformer coupling in which the self-inductances of the primary and of each halfof; the secondary can: be individually varied by: moving cores of high permeability, without affecting the coupling between the primary and either half of the secondary, for tun ing the primary and secondary and for electrically positioning the center tap of the secondary.

It is a further object of the prmentdnventiom to devise an improved transformer; ccupling em; ploying permeability tuning of the primary and secondary thus eliminating the need for variable, capacitors to make possible decreases in the physical size of the couplings and increases in, their input impedance.

It is,a.further. objectogh the. present invention tov devise. an improved. discriminaton employing, a.. transformer coupling of improved. flexibility andefficiency asstatedabove,

other objects, features. and. advantagesofthis invention will, be apparent. from thefollowing. de-.. tailed. description of, the inventionandfrom, the. drawing. in which thesingle figure. is. areprer. sentation. of. the. circuitof. adiscriminatorac: cording to the presentinventicn.

In. Fig. 1 tube I, is the. output tubeof .the inter.- mediate fr quency. amplifier of. anFM eceiyer 01 0f the sound, channel oi a. television. receiver. The.- screen gridof. tubev l. is energized over, re,. sistor 2,1rom a source of directpotential, Bel-t which, if desired, may be. connected, to. resistor. 2., over; a. decoupling network comprising a. con-i denser. 3.and anotherresistor. l, and .the-screen gridis grounded. as; to. intermediate frequency signals over byrpass. condenser. 5. Theanode of tubel iii-energized from the same sourceover resistor. 2. anda primary. 6,]. of, a: transformer coupling inserieswith the resistor. The; anodecircuit. ground return i or intermediate. frequency. signals,.is,over: primary 6 1-. and bye-pass COIL? denser 5. Primary-6, l consistsoi two portions, a.- portion 5, the self-inductanceof, which. may be varied by moving acoi'e of. highmenneability; and a portion 'i-which,,though it. is,in .series,.with portion 6,, is situated. sufiiciently. distant there,-. from,to be unaffectedby movementsoflthe, core.-. Portion Eimay be enclosed within a shields to. reduceany direct. coupling between it and other. elementsof the. transforme co plin T e sec ondary, of the transformer couplingincludestivo r halves 9' and. [6,. each. of which. is inductively. coupledto portion lot the. primary and, each of which. canbe varied in. its. seli-inductance by. movements of. a, core of high permeability. The. values of. coupling between. portion 1 and, half. 8. and. pOrtion. and half. were. e ualto each other and I arerfixed, (by. controlling, such, fa tor asthe proximity of the coupled elements and theflrelar: tive directionsof theirfluxlines), ata value selected; in. accordance with. usu l. onsiderati ns. 01. example, t s. Well known that; a desired; length for the linear portion of a discriminator characteristic can be obtained by controlling, among other things, the Q of its transformer coupling and that this, in turn is done by controlling, in addition to the values of the diode load resistors, the degree oi coupling between, the primary and the secondary. While; in certain embodiments portion? may be mounted on.

i st ble. s pporting. means. t i rmit..contr01',-

after manufacture, of the values of it coupling to halves Si and If this is not necessary in accordance with the present invention. For should these values turn out to be unequal through manufacturing inaccuracy this coupling is otherwise adjustable to effect direct and adequate compensation; as distinguished; from; adjustment of the values themselves.

The I. F. output signal from the anode of tube l is directly coupled over a direct current blocking condenser l l to the point of juncture between secondary halvesdand l8, i. e. to the point corresponding to the secondary center tap of a conventional coupling.

'Il le tuned-secondary of the coupling herein includeszla; condenser H in shunt to its series connected secondary halves. This condenser may-be madevari-able for increased flexibility. A;.di.0d61-.l3, which may be one section of a double diode I4, is connected across secondary half 9 over load resistor l5. and similarly a diode, HS is connectedfacross secondary half l9" over its inseries load resistor I'l Resistors I5 and; ll'arc connected in series to. form an output. circuit with by-pass for intermediate frequencies provided by shunt condenser E8, all of this being a well known arrangement for, discriminator circuits. The outputvoltage is taken across both' resistors and therefore, it. equals, at. any instant" offtime; the algebraic sum of voltages separately developed. acrosseach of them. The operationof discriminators is well knownand will'notbefully described'herein for the purpose of'explaining this invention.

'I'hetuned-primary of' the embodiment shown in Fig, 1 does. not include a condenser'as such; 1. e. does not include deliberately added lumped capacitance. This is possible herein'since the primary is, tuned by variations in" inductance rather than in capacitance; In other-Wards the capacitance. of this anti-resonant circuit: consists solely of the anode-to-cathode capacitance of tube I and. the stray and, d-istributedcapacitance of the circuit and this is possible since there is no need for avariable capacitor; However, where desired; for example for lowfrequency operation, a condensermay be added to the primary circuit in shunt to primary 6*; 1. This is indicatedin Fig.- l by a dotted line indication ofja condenser and connections-thereof. By omitting this, condenser theshunt capacitance of the circuit can be reduced to a minimum. It is well known that the input imped ance of an antirresonant circuit becomes higher as the shunt capacitance therecl is reduced (ofcourse, while the inductance is being appropriately increased to maintain anti-resonanceatthe same frequency) and that this, in certain circuit arran ements, increases both gain and efiiciency. The omission of lumped capacitance isparticularl suitable where, as in existing televisionand frequency modulation channels, the carrier-isofjvery high frequency and accordingly a iehs n ermed a e equencyis sel The coupling of- 1- may be adjusted as follows: First, and; in allcases, the primary istunedto the carrier frequency by adjusting theself-inductance of portion '5 and it is" allowed to remain so tuned, The remainin adjustments are made in one way-in some cases and ina diiierent way in all other cases. In cases-where the vaiues of the coupling of portion 7 to secondary halves. 9 and W are, equal, the self+inductances of the secondary halves are adjusted so that their individual values are equal'-t'hu scausing their junction to be electrically located at the center of the secondaryand so' that their combined value is correct for tuning the secondary to the same frequency as that to which the primary is tuned. In cases where the values of the coupling of portion 1 to secondary halves 9 and ill are unequal (through manufacturing inaccuracy), the self-inductances of the secondary halves are adjusted so that their individual values are appropriately unequal for displacing their junction sufiiciently from the electrical center of the secondary to compensate for the effect produced on the discriminator characteristic by the inequality of coupling, and so that their combined value is correct for tuning the secondary to the same frequency as that to which the primary is tuned. When either of these procedures is followed the end results will be that both the primary and the secondary will be tuned to the carrier frequency and that the discriminator characteristic will be linear and symmetrical.

Both halves 9 and All of the secondary and portion I of the primary may be enclosed in a grounded shield [9 to prevent undesired coupling between the secondary halves and other elements of the circuit. In addition the movable cores of each of the secondary halves 9 and It should be positioned at a sufficient distance from portion 1 and from the other secondary half so that its movements will affect only the self-inductance of its own half (not that of portion 1 or of the other half) and will not affect the coupling of portion 1 to either of the halves. Thus each movable core should have no direct significant effect on coupling at all and its effect on selfinductance should be limited to the secondary half with which it cooperates.

While the transformer coupling of the present invention is particularly adaptable to use in a discriminator, its usefulness is not restricted thereto. It may be used in a variety of circuits requiring a tuned transformer coupling with the same characteristics as those of the coupling described herein.

While it will be preferable for most embodiments to employ movable cores of high permeability for Varying the self-inductances of portions of the primary and the secondary, it is not essential that such cores be employed. For certain embodiments it will be alternately feasible to employ movable conductive slugs of the kind which reduce the self-inductance of a coil as it is brought into closed engagement therewith in stead of increasing it in the manner of a core of high permeability. The use and manner of operation of such slugs are well known and need not be further described herein. Of course, conductive slugs should only be used in circuits where the resulting increased losses and lowerings of Q can be tolerated.

Of course, the electrical location of the juncture of the two secondary halves could be controlled even in an embodiment in which the selfinductance of only one of the halves is made variable. However, in such an embodiment it would be necessary for condenser l2 to be variable so that the secondary could be tuned as desired.

What is claimed is:

1. In a frequency-modulation receiver including an intermediate frequency amplifier, a ire quency discriminator comprising: a pair of electron discharge devices each having first and second electrodes; a pair of impedance elements series connected between said first electrodes; first and second series connected inductance coils inductively isolated from each other and coupled to said amplifier to form a component of a tuned output circuit therefor; an adjustable element for varying the inductive value of said first inductance coil to tune said output circuit to the selected intermediate frequency of said receiver; a winding comprising a further pair of series connected inductance coils connected between said second electrodes and individually inductively coupled to said second inductance coil; a connection extending from the junction of said further pair of said inductance coils to the junction of said impedances; and separate adjustable elements for individually varying the respective inductances of said last-mentioned inductance coils to values having a relation effective to establish the junction of the latter coils at a selected electrical point on said winding and having a combined value effective to tune said winding to a predetermined frequency.

2. In a frequency-modulation receiver including an intermediate-frequency amplifier, a frequency discriminator comprising: a pair of electron discharge devices each having first and second electrodes; a pair of impedance elements series connected between said first electrodes; first and second series connected inductance coils inductively isolated from each other and coupled to said amplifier to form a component of a tuned output circuit therefor; an adjustable element for varying the inductive value of said first inductance coil to tune said output circuit to the selected intermediate frequency of said receiver; a winding comprising a further pair of series connected inductance coils connected between said second electrodes and individually inductively coupled to said second inductance coils with equal coupling values; a connection extending from the junction of said further pair of inductanec coils to the junction of said impedances; and separate, permeability adjusting elements for respectively varying the inductance of said last-mentioned inductance coils to a relative value effective to establish the junction of the latter coils at the electrical center of said winding and to a combined value effective to tune said Winding to said selected intermediate frequency.

3. In a frequency-modulation receiver including an intermediate frequency amplifier, a frequency discriminator comprising: a pair of electron discharge devices each having first and second electrodes; a pair of impedance elements series connected between said first electrodes; first and second series connected inductance coils inductively isolated from each other and coupled to said amplifier to form a component of a tuned output circuit therefor; an adjustable element for varying the inductive value of said first inductance coil to tune said output circuit to the selected intermediate frequency of said receiver; a winding comprising a further pair of series connected inductance coils connected between said second electrodes and individually inductively coupled to said second inductance coil with unequal coupling values; a connection extending from the junction of said further pair of inductance coils to the junction of said impedances; and separate, adjustable elements for individually, respectively permeability-adjusting of said last-mentioned inductance coils to inductances having a combined value effective to tune said winding to a predetermined frequency ammana and; having arelmtiw value; efiecflva; tm esfiahliah the JTImQtierrv 0L ns: latter: coi s: at: el ch lqal. point: o .v said; winding snlacecl; from thfi- 1% tr sal' c nter thereof by an. am unt-sufficie t to cqmn nsatez fur: he fi ct md ru ed 0127a charaet risti 0: said discr m nator bythe inszq ity of the afore-ment-ioned' coupling; values.

JOSEPH C". SPINDLER.

am Dat Kirkwood: Sept. 26; 1.939

Number 8 Nam ate QMQQIIEQT ww- Qec, 3.1, 49 311,512: V V May 20, 1941 Carlson May 27, 1941 Andrews July 31, 1945 willoughby May '7, 1946 Beardetal July 16, 1946 Fox Mar. 11, 1947 Parker Sept. 7, 1948 Sands Sept. 14, 1948 

